This invention generally relates to the field of communication systems and, more particularly, to digital communication systems that supports multiple modulation and channel coding schemes.
In wireless digital communication systems, standardized air interfaces specify most of system parameters, including modulation scheme, channel coding scheme, burst format, communication protocol, symbol rate, etc. For example, European Telecommunication Standard Institute (ETSI) has specified a Global System for Mobile Communication (GSM) standard that uses time division multiple access (TDMA) to communicate control, voice and data information over radio frequency (RF) physical channels or links using Gaussian Minimum Shift Keying (GMSK) modulation scheme at a symbol rate of 271 ksps. In the U.S., Telecommunication Industry Association (TIA) has published a number of Interim Standards, such as IS-54 and IS-136, that define various versions of digital advanced mobile phone service (D-AMPS), a TDMA system that uses a Differential QPSK (DQPSK) modulation scheme for communicating data over RF links.
Digital communication systems use a variety of linear and non-linear modulation schemes to communicate voice or data information in bursts. These modulation schemes include, GMSK, Quadrature Phase Shift Keying (QPSK), Quadrature Amplitude Modulation (QAM), etc. GMSK modulation scheme is a non-linear low level modulation (LLM) scheme with a symbol rate that supports a specified user bit rate. In order to increase user bit rate, high-level modulation (HLM) schemes can be used. Linear modulation schemes, such as QAM scheme, may have different level of modulation. For example, 16QAM scheme is used to represent the sixteen variations of 4 bits of data. On the other hand, a QPSK modulation scheme is used to represent the four variations of 2 bits of data.
In addition to various modulation schemes, digital communication systems can support various channel coding schemes, which are used to increase communication reliability. For example, General Packet Radio Service (GPRS), which is a GSM extension for providing packet data service, supports four channel coding schemes. A Convolutional Half-Rate Code scheme, CS1 coding scheme, which is the "mother" channel coding scheme of GPRS. The CS1 scheme is punctured to obtain approximately two-third rate and three-fourth rate code schemes, CS2 and CS3 coding schemes. GPRS also supports an uncoded scheme, known as CS4 coding scheme.
Generally, channel coding schemes code and interleave data bits of a burst or a sequence of bursts to prevent their loss under degraded RF link conditions, for example, when RF links are exposed to fading. The number of coding bits used for channel coding of data bits corresponds to error detection accuracy, with higher number of coding bits providing higher bit error detection accuracy. For a given gross bit rate, a high number of coding bits, however, reduces user bit rate, since coding bits reduce the number of user data bits that can be transmitted in a burst.
The communication channel typically introduces errors in sequence. In order to improve coding efficiency, the coded bits are interleaved, before transmission. The purpose of interleaving is to distribute the errors over several code words. The term perfect interleaving is used when the sequence of the received data bit errors are uncorrelated. The more less uncorrelated the received data bits are at the receiver, the easier it is to recover lost data bits. On the other hand, if interleaving is not effective, large portions or blocks of transmitted data bits may be lost under degraded RF link conditions. Consequently, error correction algorithms may not be able to recover the lost data.
TDMA systems subdivide the available frequency band into one or several RF channels. The RF channels are divided into a number of physical channels corresponding to time slots in TDMA frames. Logical channels are mapped onto one or more physical channels, where modulation and channel coding schemes are specified. An RF link includes one or more physical channels that support the logical channels. In these systems, the mobile stations communicate with a plurality of scattered base stations by transmitting and receiving bursts of digital information over uplink and downlink RF channels.
The growing number of mobile stations in use today has generated the need for more voice and data channels within cellular telecommunication systems. As a result, base stations have become more closely spaced, with an increase in interference between mobile stations operating on the same frequency in neighboring or closely spaced cells. Although digital techniques gain more useful channels from a given frequency spectrum, there still remains a need to reduce interference, or more specifically to increase the ratio of the carrier signal strength to interference, (i.e., carrier-to-interference (C/I)) ratio. RF links that can handle lower C/I ratios are considered to be more robust than those that only can handle higher C/I ratios.
Depending on the modulation and channel coding schemes, grade of service deteriorates more rapidly as link quality decreases. In other words, the data throughput or grade of service of more robust RF links deteriorates less rapidly than those of less robust RF links. Higher level modulation schemes are more susceptible to link quality degradation than lower level modulation schemes. If a HLM scheme is used, the data throughput drops very rapidly with a drop in link quality. On the other hand, if a LLM scheme is used, data throughput and grade of service does not deteriorate as rapidly under the same interference conditions.
Therefore, link adaptation methods, which provide the ability to dynamically change modulation scheme, channel coding, and/or the number of used time slots, based on channel conditions, are used to balance the user bit rate against link quality. Generally, these methods dynamically adapt a system's combination of channel coding, modulation, and number of assignable time slots to achieve optimum performance over a broad range of C/I conditions.
One evolutionary path for next generation of cellular systems is to use high-level modulation (HLM), e.g., 16QAM modulation scheme, to provide increased user bit rates compared to the existing standards. These cellular systems include enhanced GSM systems with GPRS extension, enhanced D-AMPS systems, International Mobile Telecommunication 2000 (IMT-2000), etc. A high level linear modulation, such as 16QAM modulation scheme, has the potential to be more spectrum efficient than, for example, GMSK, which is a low-level modulation (LLM) scheme. Because higher level modulation schemes require a higher minimum C/I ratio for acceptable performance, their availability in the system becomes limited to certain coverage areas of the system or certain parts of the cells, where more robust links can be maintained.
In order to provide various communication services, a corresponding minimum user bit rate is required. In voice and/or data services, user bit rate corresponds to voice quality and/or data throughput, with a higher user bit rate producing better voice quality and/or higher data throughput. The total user bit rate is determined by a selected combination of techniques for speech coding, channel coding, modulation scheme, and for a TDMA system, the number of assignable time slots per call.
Data services include transparent services and non-transparent services. Transparent services, which have a minimum link quality requirement, provide target user bit rates. A system that provides transparent communication services varies the gross bit rate to maintain a constant user bit rate with the required quality. Conversely, in non-transparent services, for example, GPRS, the user bit rate may vary, because erroneously received data bits are retransmitted. Unlike non-transparent services, transparent services do not retransmit erroneously received data bits. Therefore, transparent services have a constant point-to-point transmission delay, and non-transparent services have a non-constant point-to-point transmission delay.
A communication system may provide a data service through a number of RF links supporting different combinations of channel coding, speech coding, and/or modulation schemes. For example, the system may provide a multimedia service using two or more separate RF links that separately provide audio and video signals. Under this scenario, one of the two RF links may use HLM scheme and the other link may use LLM scheme. In order to provide a constant user bit rate in a TDMA system, lower level modulation schemes may use a higher number of time slots than higher level modulation schemes.
Moreover, digital communication systems must also select a suitable combination of channel coding and modulation schemes based on link quality. For example, for a high quality link, higher level modulation or less channel coding results in higher user bit rate, which may be used advantageously by different communication services. For example, in a non-transparent data service, user data throughput is increased. For a speech service, the increased user bit rate may be used for deploying an alternative speech coder with higher quality. Therefore, a system that supports multiple modulation and channel coding schemes should provide sufficient flexibility for selecting an optimum combination of modulation and channel coding schemes.
Conventional method for selecting an optimum combination of modulation and channel coding schemes assume that the link quality parameters are perfectly known at a given instant. Usually, these methods determine link quality parameters by measuring, at predefined instances, one or more of received signal strength (RSS) or bit error rate (BER), etc. Using these instantaneous measurements, these methods also assume that user quality as a function of link quality parameters is perfectly known for all combinations of modulation and channel coding schemes.
Because these parameters vary continuously, the mean measurement of link quality parameters do not give an accurate indication of user quality, especially after a link with a different combination of modulation and channel coding schemes is selected. One method dynamically adapts user bit rate of a TDMA system to achieve optimum voice quality over a broad range of channel conditions. This system continuously monitors link quality by making instantaneous measurements of a RF link's C/I ratio. The system dynamically adapts its combination of modulation and channel coding schemes and the number of assignable time slots to optimize voice quality for the measured conditions. In addition, the system determines cost functions to derive at a cost of using RF links with different modulation and coding schemes to improve voice quality.
User quality, however, varies considerably with variations in link quality parameters. FIG. 1 shows link performance of two modulation schemes, i.e., QPSK and 16QAM schemes, which are exposed to three channel conditions: an Average White Gaussian Noise (AWGN) channel condition, a fast Rayleigh Fading channel condition, and a slow Rayleigh fading channel condition. In FIG. 1, link performance is expressed in terms of BER. For a given C/I ratio, the AWGN channel provides the best performance, due to the lack of fading dips. In fast Rayleigh fading channel, where fading varies fast enough to make effective use of interleaving, link performance is degraded compared to the AWGN channel. In slow Rayleigh fading channel, where fading varies slowly such that interleaving is not effective, the worst link performance is obtained. Conventional methods use mean C/I ratio to determine the channel condition. As shown in FIG. 1, however, mean C/I ratio for different channel conditions may be the same, when link performance under different combination of modulation and channel coding schemes may be quiet different. Therefore, more information is needed to accurately estimate link performance, if different combinations of modulation and channel coding is used.
An additional factor affecting user quality is time dispersion. Receiver equalizers can not effectively handle large time dispersions. As a result, link performance degrades, even when C/I ratio distribution remains the same. Accordingly, mean measurements of C/I ratio, BER or time dispersion alone are not sufficient for estimating performance of a selected link. Therefore, there exists a need for an effective link selection method in systems that support various modulation and channel coding schemes.